Abstract: Equipment Replacement is a part of maintenance given to a system to run effectively and efficiently. A lot of researchers have carryout much research work to determining the specific time of replacement through various models. Theory of replacement is normally concern with the problem of replacement of machines, man and decreased efficiency, equipments due to deterioration , break down or failure. Base on several research or studied by researchers on Replacement problem which is an essential theme in management science and operation research .Many writers have assumed that the framework in the working situation where degradation or wear of a unit can be measured and have formulate various models for replacement .